Mercedes has fallen behind because of "distractions" rather than the improvement of title rival Red Bull's Honda engines.
That is the view of Ferrari team boss Mattia Binotto, who rejects Mercedes' argument that the newly-fitted Honda specification took a major leap forward at Paul Ricard.
"Our GPS data shows that the performance of the Honda engine corresponds with its level at the start of the world championship," the Italian said.
"I can imagine that they used slightly reduced power in the first races because they were worried about reliability. But in terms of performance, an update is not allowed at all in the regulations."
Therefore, Binotto said he applauds the "great job" done by Red Bull so far in 2021.
"They are now reaping the success that is due in part because a stable team has worked together for years," he explained.
"On the other hand, we are seeing significant changes at Mercedes in terms of staff - either departures or employees who have other tasks."
He is probably referring to technical boss James Allison, who is moving into a less intense role this month.
But Binotto also mentioned seven time world champion Lewis Hamilton, and the fact that Mercedes has "a driver who wasn't confirmed until February 2021".
"I find that to be quite a lot of distractions, which surely contributed to the situation they are in today," he added.
